Hey guys, new to site, I recently bought a 2007 400ex it did not run rebuilt motor piston rings, timing chain and tensioner, new valves, cleaned carb, it started and runs, but runs awful backfires bogs, it idles ok but once you give it gas it runs bad. It has a fmf power core 4 slip on so I looked at jets it was never re jetted so I've gone through from 150 to 158 main and 42 pilot still having issues I've made multiple adjustments to fuel screw I'm at a lost. And the plug comes out very black. Would the packing in the exhaust do this? Any ideas would be great thanks.